Typing notification for "more files coming your way"

It's surprisingly common that someone on their phone/desktop sends 3+ large images into a room, at least for me. Would be nice to have a typing-like notification to say that there's more files on the way to avoid accidentally sending a message between two files, forever losing that message.

No files, tests, or entry points are named in the issue. Start by tracing how typing indicators and multi-file sends are handled in the Element clients; done means a recipient is warned that additional files are still coming without losing an intervening message.

Written by the indexing model from the issue text.